In this field, the term “User Equipment, UE” is commonly used and will be used in this disclosure to represent any wireless terminal or device capable of radio communication with a cellular network including receiving downlink signals transmitted from a serving radio node and sending uplink signals to the radio node. For example, the term User Equipment, UE could be exchanged by the term “wireless device”. Further, the term “radio node”, also commonly referred to as a base station, e-nodeB, eNB, etc., represents any node of a cellular network that can communicate uplink and downlink radio signals with UEs. The radio nodes described here may, without limitation, include so-called macro nodes and low power nodes such as micro, pico, femto, Wifi and relay nodes, to mention some customary examples. Throughout this disclosure, the term “eNB” is often used but can be exchanged by the term radio node.
Link adaptation in systems according to Long Term Evolution, LTE, is based on adaptive modulation and coding, which controls data rate by adaptively adjusting the modulation scheme and/or channel coding rate according to the radio-link conditions. In this procedure, the Modulation and Coding Scheme, MCS, adopted for Physical Downlink Shared Channel, PDSCH, transmission must be indicated in downlink MCS signaling by the serving radio node to the UE. By uplink signaling, the UE informs the radio node about corresponding radio-link, i.e. channel, conditions through Channel Quality Indicator, CQI signaling, including sending CQI reports to the radio node.
This is generally illustrated in FIG. 1 in which a radio node 100 of a cellular network is serving two UEs denoted UE1 and UE2. In this example, UE1 and UE2 both report quality measurements made on the channel used by sending CQI reports to the radio node 100 which selects a suitable MCS for each UE based on their CQI reporting and signals the selected MCS to the UEs, respectively. Link adaptation is made in this way for individual UEs on a dynamic basis since the radio-link conditions may change rapidly. The selection of a suitable MCS can thus be made individually for each UE.
In current LTE systems, the set of available modulation schemes for both downlink and uplink includes Quadrature Phase-Shift Keying, QPSK, 16 Quadrature Amplitude Modulation, QAM, and 64QAM, corresponding to two, four and six bits carried per modulation symbol, respectively. In this field, the number of bits carried per modulation symbol is usually referred to as the modulation order, Qm.
In brief, the serving radio node selects a suitable MCS based on CQI reporting from the UE and signals the selected MCS to the UE with reference to a predefined MCS index table which is known to the UE. The MCS index table maps MCS indices to modulation order and a Transport Block Size, TBS, index. Further, the UE determines a CQI value based on signal measurements and the CQI is signaled from the UE to the radio node with reference to a likewise predefined CQI index table which maps CQI indices to modulation forms and code rates. In this description, the term “modulation form” is used for short to indicate a modulation format, method or scheme.
In LTE, especially for scenarios with good channel conditions where the Signal-to-Interference-and-Noise Ratio, SINR, is high, e.g. in small-cell environments where the UE is close to its serving radio node, a straightforward means to provide higher data rate for the UE with given transmission bandwidth is to use higher-order modulation that allows for more bits of information to be carried per modulation symbol, as compared to the modulation schemes mentioned above where the highest possible data rate is provided by 64QAM carrying six bits per modulation symbol. However, it is a problem that the control signaling schemes, methods, formats or protocols of today do not support any modulation with higher order than six bits per symbol, as in 64QAM. It is also a problem that additional control signaling would be required between the UE and the serving radio node if higher data rate is to be achieved by using higher-order modulation.